Thanks for providing the pointer. Do you have any preliminary result or goal (e.g. the replacement ratio) of the optimization? Is it going to replace all ARC operations with non-atomic ones for single-threaded applications?
On Wed, Nov 30, 2016 at 8:50 PM Roman Levenstein <rlevenst...@apple.com> wrote: > On Nov 30, 2016, at 6:25 PM, Jiho Choi via swift-dev <swift-dev@swift.org> > wrote: > > Thanks for clarifications. I have a couple of follow-up questions. > > 1. Could you please provide more information (e.g. source code location) > about the optimization applying non-atomic reference counting? What's the > scope of the optimization? Is it method-based? > > > The optimization itself is not merged yet. But all the required machinery, > e.g. non-atomic versions of the ARC operations, special non-atomic flag on > SIL instructions, etc is in place already. > > As for the prototype implementation, you can find it here, on my local > branch: > > https://github.com/swiftix/swift/blob/30409865ff49a4268363cd359f82f29c9a90cce8/lib/SILOptimizer/Transforms/NonAtomicRC.cpp > > > 2. Looking at the source code, I assume Swift implements immediate > reference counting (i.e. immediate reclamation of dead objects) without > requiring explicit garbage collection phase for techniques, such as > deferred reference counting or coalescing multiple updates. Is it right? > If so, is there any plan to implement such techniques? > > > Yes. It is a correct understanding. > Different extensions like deferred reference counting were discussed, but > there are no short-term plans to implement it anytime soon. > > -Roman > > > On Wed, Nov 30, 2016 at 11:41 AM John McCall <rjmcc...@apple.com> wrote: > > On Nov 30, 2016, at 8:33 AM, Jiho Choi via swift-dev <swift-dev@swift.org> > wrote: > Hi, > > I am new to Swift, and I have several questions about how ARC works in > Swift. > > 1. I read from one of the previous discussions in the swift-evolution list > ( > https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160208/009422.html) > that ARC operations are currently not atomic as Swift has no memory model > and concurrency model. Does it mean that the compiler generates non-atomic > instructions for updating reference counts (e.g. using incrementNonAtomic() > instead of increment() in RefCount.h)? > > > No. We have the ability to do non-atomic reference counting as an > optimization, but we only trigger it when we can prove that an object > hasn't escaped yet. Therefore, at the user level, retain counts are atomic. > > Swift ARC is non-atomic in the sense that a read/write or write/write race > on an individual property/variable/whatever has undefined behavior and can > lead to crashes or leaks. This differs from Objective-C ARC only in that a > (synthesized) atomic strong or weak property in Objective-C does promise > correctness even in the face of race conditions. But this guarantee is not > worth much in practice because a failure to adequately synchronize accesses > to a class's instance variables is likely to have all sorts of other > unpleasant effects, and it is quite expensive, so we decided not to make it > in Swift. > > 2. If not, when does it use non-atomic ARC operations? Is there an > optimization pass to recognize local objects? > > 3. Without the concurrency model in the language, if not using GCD (e.g. > all Swift benchmark applications), I assume Swift applications are > single-threaded. Then, I think we can safely use non-atomic ARC > operations. Am I right? > > > When we say that we don't have a concurrency model, we mean that (1) we > aren't providing a more complete language solution than the options > available to C programmers and (2) like C pre-C11/C++11, we have not yet > formalized a memory model for concurrency that provides formal guarantees > about what accesses are guaranteed to not conflict if they do race. (For > example, we are unlikely to guarantee that accesses to different properties > of a struct can occur in parallel, but we may choose to make that guarantee > for different properties of a class.) > > 4. Lastly, is there a way to measure the overhead of ARC (e.g. a compiler > flag to disable ARC)? > > > No, because ARC is generally necessary for correctness. > > John. > > _______________________________________________ > swift-dev mailing list > swift-dev@swift.org > https://lists.swift.org/mailman/listinfo/swift-dev > > >
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